The techniques generally used to mold thermosetting resins include compression molding and transfer molding. Compression molding involves placing a resin or polymer sample directly into a mold cavity designed to receive the material. Thereafter, the molding compound is liquefied through a heating process before being formed into the shape of the mold as the mold closes upon itself. After a cooling period, or "curing", and removal from the mold, the resin or polymer material corresponds to the shape of the mold used.
Transfer molding involves placing the molding compound into a separate transfer pot or chamber that applies heat sufficient to liquefy the polymer material. Then, through the application of a transfer plunger, the liquefied polymer, or some set amount of it, is forced to flow into the selected mold cavity, where the mold is then closed so that the polymer is molded and later cured. After curing the mold is opened and the molded polymer form is removed.
In the manufacture of epoxy materials, or other resinous compounds, minor variations in formulation or quality of the ingredients can significantly affect certain qualities of thermoset formulations. Among the qualities that can be dramatically affected are cure time, optimum time for heating prior to injection, speed with which the material should be injected, size of a mold which can be properly filled, integrity of the molding material both during and after molding, and the optimum time to remove molded material from a mold. To evaluate these various qualities of epoxy materials both during the manufacturing process and immediately prior to use, a convenient testing operation is needed.
As computer simulation or Computer-Aided Engineering (CAE) techniques are becoming more essential tools to ensure successful polymer processing, the need for accurate characterization of rheological properties of reactive thermoset materials also becomes very critical. CAE techniques can be used to enhance the quality of industrial polymer molding. Other possibilities for use include the overall optimization of polymer processing, and in the actual design of molds to be used for a specific application. For the proper use of these CAE programs, however, the properties of the polymer materials to be used in the production process must be determined very accurately. Existing instruments and techniques for measuring viscosity at low-temperature and low shear-rate are adequate but most manufacturing processes are under very high shear-rate conditions. The difference in these conditions and the data generated therefrom can quickly lead to problems when quick setting resinous materials are molded at high-shear rates in reliance upon data generated at low shear rates and then extrapolated.
Among the properties that must be determined for polymer materials, viscosity is most important. Viscosity is a measure of the energy dissipation and consequent generation of stress in a fluid, often described, and measured, as flow resistance. The viscosity as a function of temperature of a resinous material to be used in the molding process plays a very important role in the determination of the properties of the eventual product. The viscometer disclosed herein uses quantitative measurements of viscosity, not only as a function of temperature, but also shear rate and the rate of cure in order to determine the exact characteristics of the sample compounds.
There are several methods that are currently being used for the rheology measurement of fast-reacting thermosets. Among them, the parallel-plate viscometer is the most widely used device (Halley and Mackay, 1996). A typical parallel-plate viscometer has two parallel circular plates separated by a specified distance, with a sample inserted between the plates. One plate is connected to a motor which rotates the plate, while the other plate is connected to a transducer, whereby the torque is measured. The rheology of the sample is determined from the applied rotation speed and the measured torque.
Although the parallel-plate viscometer, as found in the prior art, has several advantages, it also has significant limitations. The advantages include small sample size, the ease of cleaning the test device, and the capability of testing at various modes. Perhaps the most significant limitation demonstrated by parallel-plate viscometers is that the range of shear rates of the sample polymer which can be measured in this type of viscometer are rather small. This limitation is similar to other instruments in the prior art whose design did not allow accurate determination of physical characteristics of the thermoset materials at extreme conditions, which are generally the type of conditions most often encountered in today's industrial processes.
Since shear rates in extreme conditions, i.e., high shear rates, cannot be measured by the parallel-plate type viscometers generally in use, there has been an effort made to use the information collected at low shear rates to generate more complete data curves by extrapolation. Since this data was based on extrapolation, it was not, and could not be made, entirely reliable.
Another point that should be made about the inadequacy of the data generated through the use of the parallel-plate viscometer is that the measurement of viscosity of fast-curing thermoset materials at high temperatures is not practical or accurate due to the significant time delay experienced in heating the sample. Simply put, the time it takes for the materials to reach the equilibrium temperature with this instrument is often longer than it takes for these materials to "set" and begin their cure. It is these newer and faster reacting materials that are becoming more popular in manufacturing today, and for them the data generated by the parallel-plate viscometer is not as accurate as it should be because the extrapolation of data from low temperature to high temperature is required.
When using known technologies to obtain the viscosity at shear rates and temperatures that more accurately reflect the extreme conditions of heat and pressure that are routine in current industrial processing today a new process is needed. Too much reliance on extrapolation can lead to errors in both calculation and production.
Another popular method for viscosity characterization of resinous materials, and specifically Epoxy Molding Compounds (EMC's), is through the use of the "spiral flow test" (Gonzalez et al., 1992). In the spiral flow test, a sample polymer compound is forced through the entrance and into a spiral shaped mold cavity. This usually occurs at approximately 300.+-.5.degree. F., but can be altered to suit the specific polymer material used. Generally, there is an approximate pressure of 500 psi. applied to a sample, as calculated from the surface area of the piston used. The distance the compound flows into the spiral mold is then measured. The distance of this flow is primarily a function of the viscosity of the compound when in liquid state, and the time which it requires to "set" or gel.
Typically molds used in industry have a runner or trough-like cavity leading from the injection opening to a gate which gate is in some way connected to the main molding cavity. The spiral flow test, however, creates the equivalent of a spiral runner. This extruded ribbon of sample or "runner" is a polymer which has no equivalent in the vast majority of typical industrial molding products. Accordingly, the flowing tip of molding material in a spiral flow mold does not share the same flow characteristics or shear rates of the same or similar material used in a conventional mold to produce typical products. The excessive length of the runner and the varying radius of the spiral also contribute to the problem of using the data generated from the spiral flow test as indicative of the flow characteristics of actual mold products.
Also, the spiral flow test fails to conveniently allow determination of optimal times for injection and curing of different epoxy molding materials. While the utility of the spiral flow test is demonstrated in its simplicity, there are problem in addition to the problems already outlined. These problems include the fact that this method requires extensive numerical simulations to back out the viscosity parameters from the melt-front trace and/or pressure trace measurements. Compounding this is the fact that this data often does not produce converged values, a factor which will often then lead to erroneous results.
An alternate method used to measure the viscosity of thermoset materials is through the use of capillary instruments. Capillary instruments share the disadvantage of requiring a significant dwell time which makes measurement at high temperatures difficult, as well as the difficult and costly cleanup of the setting materials used (Blyler et al., 1986).
As discussed above, none of the prior devices or methods mentioned above is able to obtain data at the whole range of the normally utilized industrial processing ranges.
The apparatus and associated methods of the present invention are designed to quantifiably measure the rheology of thermoset materials as accurately as possible throughout the entire range of shear rate, temperature, and degree of cure often encountered in modern industrial processes. Thus, the basic design of this device is itself a departure from the prior art. Moreover, this device improves upon the prior art through its ability to allow the accurate and reliable testing of extremely fast reacting or crystallizing materials, quite beyond the reach of the prior art, as already discussed.